Natural intervertebral spinal discs serve, among other things, as cushions and shock absorbers for columnar spinal loadings. In addition, they serve to preserve the spacing of vertebrae necessary for the mechanical integrity of channels which protect the spinal cord and the nerve branches therefrom which innervate various portions of the human body. Frequently, however, accident trauma, degenerative disc disease, and other pathogenesis can compromise all or a portion of the disc's ability to provide mechanical support and integrity.
In such cases, surgical intervention is usually necessary. Surgery sometimes involves only partial laminectomy, but most often the surgeon must remove a portion of the disc so large that replacement of its mechanical, or at least its separative, function is necessary.
In these cases, the most frequent surgical intervention is to attempt to achieve bony fusion of the vertebrae through the space from which tissue has been excised. The process involves inducing fusion of adjacent vertebrae, that is, the joining together of two or more vertebrae by a continuous bridge of new bony tissue. Most frequently an implant serves as a substrate for bone growth.
Many different approaches to fusion have been tried in the last several decades. Examples are metallic and ceramic spacers which are designed to allow the growth of new bone. Prior art devices of this sort have shown very mixed results.
The most promising approach for some time has been the use of human bone as an implant. When this technique succeeds, new bone grows into the implanted bone and provides robust mechanical support between the two compromised vertebrae. Initially it was thought that use of bone from the patient's own body was the best approach. Specially shaped bone transplants from elsewhere in the body, such as from the iliac crest, have been common.
However, use of autograft bone has serious limitations. The use of the patient's own bone to fill the space is less than optimal in that bone obtained from the patient requires an additional surgery site, additional healing and risk of infection, and is available only in relatively small amounts. The latter disadvantage is especially acute when more than one disc must be replaced or when diseased vertebrae themselves must be replaced.
An alternative that readily suggests itself is use of bone from cadavers. Such donor bone, also referred to as allograft bone, is used after preparation that minimizes the likelihood of infection and of immune system rejection by the patient. Cancellous bone is frequently preferred for implantation, because the interstitial spaces allow the ready flow of blood through the implant, which facilitates the growth of new bone tissue. However, cortical bone is also used. The allograft implant must also be prepared by shaping it to conform to the space in which it is to be implanted and wherein it will provide strength and stability to the spinal region from which the damaged or diseased disc has been removed.
Various shapes of bone implants have been used. Roughly circular cylindrical dowels which are inserted horizontally constitute the most common shape in the patent literature but a significant number of other shapes have been tried.
Allograft bone must be shaped into special configurations according to the specific type of fusion graft deemed desirable. For example, the use of a dovetail fusion graft, as disclosed in Nicholson, et al. U.S. Pat. No. 6,096,080, means that an appropriately sized piece of allograft bone must be cut from the donor bone. The most effective use of the tissue donated for specialized bone graft shapes requires that the bone-cutting process must be reasonably efficient and very accurate.
In addition, much allograft bone of interest has specific physical properties which must be taken into account in designing machinery to produce implants from it. In particular, cancellous or trabecular bone has a cellular structure which makes such bone difficult to handle. Allograft bone used for implantation must be treated chemically and physically in order to minimize the risk of transmitting agents of disease or substances which could trigger an immune rejection by the patient. These treatments often include processes as harsh as boiling. As a result, cancellous or trabecular bone in particular, which is frequently of the most interest for spinal implants, is left in an amorphous, soft state which has been characterized as somewhat like a hard boiled egg. Cutting such a substance cleanly and precisely is obviously a challenge. Prior art or obvious methods of shaping such a substance would most likely produce a mushy or crumbled unusable mass.
There is minimal prior art relevant to the tools and machinery for the precision fashioning of allograft implants. In one recently published international application, Shimp and Morris, WO 01/49333 A2, published Jul. 12, 2001, disclose an apparatus for producing bone dowels from allograft long bones. This apparatus relies on the use of a cutter equivalent to a hole saw to core a blank dowel out of, for example, a femur. The principles of operation of that device are very different from those of the present invention. Also, in a series of patents, Bonutti, U.S. Pat. No. 6,132,472, issued Oct. 17, 2000, U.S. Pat. No. 5,888,219, issued Mar. 30, 1999, U.S. Pat. No. 5,662,710, issued Sep. 2, 1997, and U.S. Pat. No. 5,545,222, issued Aug. 13, 1996, disclosed use of a tissue press to compress tissue grafts and composites of tissue for implantation. As these inventions generally relate to maintaining the to-be-grafted tissue in a living condition until implantation, they are generally related to autograft rather than allograft formation. In any event the principles of operation are different from those of the current invention.
Accordingly, one object of the present invention is to provide an apparatus by which to cut allograft spinal fusion implants having standard shapes in multiple sizes from donor bone. A further object of the current invention is to provide an apparatus which can shape bone in a manner consistent with the delicate structure and consistency of cancellous or trabecular bone. Another object of the present invention is to provide an apparatus by which to cut donor bone accurately, with minimum waste, and to provide for easy maintenance, rapid blade changes, and rapid cleanup between batches. Another object of the present invention is to provide an apparatus that provides for an efficient, repeatable means to cut shaped implants in standard shapes and sizes from donor bone. Another object of the present invention is to provide an apparatus that provides a way to cut dovetailed implants in standardized sizes from donor bone. And yet a further object of the present invention is to provide an apparatus by which a single operator can rapidly and safely cut multiple bone implants from a single segment of donor bone.